Microvillus dense bodies (MVDB) are electron dense plaques intimately related to the inner lamina of the microvillus plasma membrane in man and rat. These structures may be associated with calcium binding or transport because they are increased in size and density in a hypoparathyroid child receiving vitamin D2 and calcium lactate. The existence, distribution and function of MVDB have not been described in the literature available to me. I have demonstrated that MVDB are present in rats. I now propose a thorough study of the distribution of MVDB in the normal rat and in young rats on low, normal and high calcium diets, during refeeding after a period on low calcium and during vitamin D deficiency and repletion. This work will 1) firmly establish the morphologic identity of MVDB by electron microscopy; 2) define the histochemical properties of MVDB in common electron microscopy fixatives and "stains"; and 3) determine the number and distribution of MVDB during calcium absorption in calcium deficient or saturated rats and calcium absorption in vitamin D deficient and treated rats.